The falling film evaporation and the effect of cross vapor stream in falling film evaporator with triangular tube bundle using R123 was experimentally investigated. The variations of local and bundle average heat transfer coefficients were described. The tube bundle consisted of 4 x 3 (column x row) of triangular horizontal copper tubes. The tests without vapor effect were conducted with nominal heat fluxes of 20 to 60 kWm(-2), saturation temperatures of 6 to 16 degrees C and film flow rates of 0.016 to 0.18 kgm(-1) s(-1). Cross vapor stream effect experiments were operated at three heat fluxes of 20, 30 and 40 kWm(-2) and two film flow rates of 0.035 and 0.07 kgm(-1) s(-1), and the vapor velocity at the narrowest interstice in the tube bundle varies from 0 to 5.0 ms(-1). It is indicated that the heat transfer is seriously influenced by the bundle effect while less affected by changing of saturation temperature. With the increase in vapor velocity, the heat transfer performance is generally weakened; the cross vapor stream has a strong influence on falling film evaporation of R123. (C) 2020 Elsevier Ltd and IIR. All rights reserved.

The influence of downward vapor stream on the falling film evaporation with nucleate boiling outside a horizontal enhanced tube bundle arrayed in a vertical column is experimentally investigated. The variations in local and bundle averaged heat transfer coefficients with vapor velocity are presented. To simulate the practical condition in a falling film evaporator, the flow passages of the liquid/vapor are constructed using two quartz glass sheets and two columns of dummy half tubes. The test ranges are saturation temperature of 6 degrees C, film flow rate of 0.035-0.1 kgm(-1)s(-1), heat flux of 20, 30, 40 kWm(-2). It is found that (1) the downward vapor effects are either positive or negative depending on the tube position, film flow rate, heat flux and vapor velocity; (2) the bundle effect becomes stronger under the effect of downward vapor stream; (3) the downward vapor has a significant effect on the heat transfer of the lower tubes than the upper tubes; and (4) the vapor stream effect on the bundle averaged heat transfer coefficient varies with the levels of film flow rate and heat flux. The experimental results provide significant guidance in the design of the vapor escape channel in a falling film evaporator. (C) 2018 Published by Elsevier Ltd.

The effect of vapor flow on the falling film evaporation of refrigerant R134a outside a horizontal tube bundle is investigated with an experimental approach. The test space is a cube with a rectangular cross section of 0.575 m (length) x 38.8 mm (width). The tube bundle was 3 x 6 (columns x rows) of staggered horizontal finned tubes made of copper. The longitudinal tube pitch is 22.5 mm and the transverse is 19.9 mm. The external fin density of test tube is 45 fpi (fins per inch), and outside diameter is 19.05 mm. The vapor flow velocity can be adjusted in the range of 0-3.1 m/s. Liquid falling film flow rate ranges from 0.07 to 0.2 kg/m.s. Experiment is firstly conducted at saturation temperature of 6 degrees C without the effect of additional vapor flow at the heat flux of 20, 60, 100 and 180 kW/m(2) (for the first tube row). Vapor flow effect experiment was carried out at three heat fluxes 20, 40 and 60 kW/m(2). It is found that falling film flow rate is an important factor to influence the evaporating heat transfer coefficient. With the effect of vapor flow, both positive and negative effects are observed as the increment of vapor velocity. Positive effects are predominant for the two tubes in the top positions and higher vapor velocity. (C) 2015 Elsevier Ltd. All rights reserved.

Enhancing convective heat transfer is important for improving performance of heat exchangers. We studied the enhancement of heat transfer in a gas-solid suspension flow wherein the solid particle motions were controlled using an electric field. In the experiments, hollow glass particles suspended in air flowed vertically upward in a channel confined by parallel-plate electrodes, one of which served as a heat transfer surface. Particle trajectories, temperature profiles in the airflow, and heat transfer rates were measured. A theoretical study was also performed by considering the particle equations of motion, electric charge transfer at the walls, and heat exchange between particles and the gas phase using the particle source in cell model. As the results, we found that Coulomb forces acting on particles caused alternating one-sided motion in the flow direction through contact charging on the wall electrodes. Thus, particles repeatedly collided with both channel walls. Hence, heat transfer was enhanced, primarily due to heat transport by particles across thermal boundary layer at the heated wall. The simulation results of heat transfer rates were compared with the experimental results, and show quantitatively good agreement. On the basis of the results, the optimum particle diameter for enhancing heat transfer was determined by imposing the condition that the thermal relaxation time of a particle is equal to the contact-charging time of the particle on the wall. (C) 2015 Elsevier Ltd. All rights reserved.

Refrigerant mal-distribution in a distributor located at the inlet of the heat exchanger used for air conditioning systems plays an important role in the heat exchanger performance. The distribution performance at the distributor is greatly affected by flow conditions as well as geometrical parameters of the distributor. To clarify the distribution characteristics, it is essential to know flow rates of both liquid and vapor state at every branch tube after distribution. In other word, the information of the quality and the flow rate at the every branch tube are required. Acquiring this information, however, requires quite complicate experimental setup so far. This paper proposes a quite simple test method, however, allowing to obtain the value of quality and the flow rate of the refrigerant at the every branch tube of the distributor by measuring pressure drop and heat transfer rate of the branch tubes after distribution. By using the proposed evaluation method, the optimization for the geometrical parameters of the distributor was conducted to reduce the refrigerant mal-distribution and an optimized distributor is suggested. It is confirmed that the optimized distributor greatly reduces the mal-distribution of the refrigerant over the every branch tube regardless of the flow conditions. Meanwhile, by the flow visualization in the distributor, it is observed that certain amount of liquid refrigerant is stayed and swaying unstably at the bottom of the distributor. It is supposed that the liquid refrigerant behavior in the distributor great affects the distribution performance.

This paper presents a new method for predicting evaporating and condensing heat transfer coefficients for refrigerants flowing inside a new type herringbone heat transfer tube which is developed for the use of an air-conditioning machine. Not until recently has there been sufficient quantitative explanation about the heat transfer enhancement mechanism of this tube, the present prediction assumes that the heat transfer coefficient is simply governed by the circumferential local thickness of an unique liquid film layer formed inside the tube. The developed semi-empirical prediction method agrees with the experimental data within the deviation of 30% for both evaporation and condensation.